Photomultipliers (PMT: Photo-Multiplier Tube) have conventionally been known as a photosensor. A photomultiplier comprises a photocathode for converting light into electrons, a focusing electrode, an electron multiplier section, and an anode, which are accommodated in a vacuum envelope. When light is incident on the photocathode in the photomultiplier, photoelectrons are emitted from the photocathode into the vacuum envelope. The photoelectron is guided to the electron multiplier section by the focusing electrode, and is multiplied in a cascading manner by the electron multiplier section. As a signal, the anode outputs electrons having arrived thereat among those multiplied (see the following Patent Documents 1 and 2).    Patent Document 1: Japanese Patent Publication No 3078905    Patent Document 2: Japanese Patent Application Laid-Open No. HEI 4-359855